Gaining weight when you don’t want to is so common in Western societies that it is almost the rule rather than the exception. Most of the time the cause is just what you think it is — we eat more calories than we burn up during our daily activities, and we store the excess calories as fat. However, there are indeed several other potential causes for unintentional weight gain.

If you are gaining weight and you genuinely don’t know why, here are eight causes of unintentional weight gain you should be aware of.

1 - Diet and Eating Habits

This, obviously, is the big one. And while it’s true that the number of calories we store as fat can be reduced to a simple equation (namely, calories consumed minus calories burned equals calories stored or lost), human physiology makes gaining or losing weight a bit more complicated than that.

To our bodies, food does not merely represent calories and nutrition; food is also information. The kind of food we eat tells our bodies something about our environment, and in response to that information our body changes several things. It changes the way we handle and store the calories we consume; it adjusts whether our appetites are stimulated or suppressed; and, finally, it slows or speeds up our metabolism. This means, among other things, that what type of food we eat also has an impact on our weight, aside from how many calories we eat.

For instance, consider what happens when we eat a lot of carbs with a high glycemic index. We boost our insulin levels. We can think of insulin as fat’s jailer — it locks up fat in our fat cells, and refuses to release it, and prevents us from burning it off. By avoiding carbs (especially the high glycemic kind) we can reduce our insulin levels, and help mobilize our “incarcerated” fat.

Eating patterns can also affect weight gain. Eating five smaller meals a day, instead of two or three larger meals, has been associated with a reduced risk of becoming overweight. Eating breakfast regularly is also thought to reduce the risk of weight gain. On the other hand, eating at night before bedtime has been associated with obesity (and sleep disturbances).

Keep in mind that as we age, our bodies change how we handle calories, and we tend to gain weight far more easily than we did in our carefree youth. This weight gain often occurs without any discernible change in our diets or activity levels — so it is “unexplained.”

If you're looking to lose the weight you unintentionally gained, striking the right calorie balance is key. Enter your information into the weight loss calorie goal calculator below to learn how many calories you should be eating for weight loss.

2 - Insufficient Physical Activity

Living a sedentary lifestyle is strongly associated with weight gain. And consuming lots of electronic and social media is the sedentary behavior that seems to most increase our risk of diabetes and obesity. Regular exercise not only burns calories directly, but it also can jazz up our metabolism so we burn more calories in between exercise sessions. Read about how much exercise you really need.

3 -Sleep Deprivation

Getting less than 7 hours of sleep per night is associated with gaining weight. The reason for this, at least partially, is that restricting sleep increases the appetite suppressing hormone leptin, and increases the appetite stimulating hormone ghrelin. So, when we are sleep deprived we are hungrier and we eat more.

4 -Smoking Cessation

Smoking cessation, while a major boon to health, can lead to weight gain. In fact, experts suggest that smoking cessation programs should always include dietary and exercise components to prevent excess weight gain.

5 - Drugs

Several prescription medications have been associated with weight gain, including:

Several drugs used to treat psychiatric disorders such as schizophrenia, depression, and bipolar disorder
Steroids
Birth control pills
Some diabetes drugs
Some drugs used for seizure disorders

If you are being treated for any of these problems with prescription medication, talk to your doctor about whether your medicine might be contributing to your weight gain.

6 - Endocrine Disorders

Several disorders of the endocrine system often lead to weight gain. These include:

You can check with your doctor to see whether you should be evaluated for any of these problems.

7 - Cardiac Disease

Heart failure is the result of several kinds of cardiac disease, and it is associated with weight gain because of fluid retention. While classically people with heart failure develop edema in the legs, as many as 10 - 12 pounds of fluid can be retained in the body without producing visible edema.

8 - Other Medical Problems

Several other medical problems can produce weight gain by causing fluid retention. For instance, kidney disorders, especially nephrotic syndrome, can lead to significant weight gain from edema. Similarly, liver problems such as cirrhosis can produce fluid retention, especially in the abdomen (a condition called ascites). Lymphedema, fluid retention caused by blockage in the lymphatic system, can also cause weight gain.

A Word From Verywell

For people who have unexplained weight gain the most common cause, by far, is related to poor dietary habits and suboptimal physical activity.

But if you are gaining weight for no apparent reason and you believe you are eating reasonably and getting a fair amount of physical activity, you should consult with your doctor about whether you might have one of the other potential causes of weight gain.